Northern South America: Northwestern Brazil

The Purus-Madeira moist forest region lies in the center of the Brazilian Amazon south of the Amazon river on the interfluvial plain between the Purus and Madiera rivers. The region extends to the southwest reaching the lowest foothills of the Carauari arch, an ancient zone of uplift. The terrain here is almost uniformly flat, being entirely on the low Amazon basin, and the vegetation are seasonally inundated tropical lowland rainforests. These lowland plains are dissected by large rivers characterized by endless meanders, frequent oxbows, and thousands of tiny streams, all of which flood annually. This hot and humid region receives on average 2500mm of precipitation per year, and elevations range from 20-60m above sea-level.

Scientific Code

(NT0157)

Ecoregion Category

Neotropical

Size

67,200 square miles

Status

Relatively Stable/Intact

Habitats

Description Location and General DescriptionThe Purus-Madeira moist forest ecoregion lies in the center of the Brazilian Amazon, south of the Amazon River on the interfluve between the Purus and Madeira Rivers. The ecoregion extends to the southwest, reaching the lowest foothills of the Carauari Arch, an ancient zone of uplift. Thus, the ecoregion lies entirely on the low Amazon Basin whose soft sediments emerged relatively recently, during the late Tertiary (2-5 million years ago). The terrain is mostly a uniform, flat plain dissected by large rivers characterized by endless meanders, frequent oxbows, and thousands of tiny streams, all of which flood annually. These flooded forests are described in the Purus Várzea ecoregion. The larger streams that cross the ecoregion in the south include the Rio Preto, Rio Ipixuna, Rio Itaparaná, Rio Mucuim, and Rio Jari. This hot and humid tropical ecoregion receives on average 2,500 mm of precipitation per year; most months receive 200 to 300 mm, while July is the driest month with 100 mm on average. Temperatures over the year average 26° to 27° C with little monthly variation. The soils of this ecoregion are variable, alternating between sandy podzols and hydromorphic clay soils. Both have limited nutrients and tend to be acidic. Elevation of this terrain is between 20 and 60 m.

The Purus-Madeira moist forest ecoregion is characterized by high biodiversity and endemism in the flora and fauna. The ecoregion is almost entirely forested with evergreen tropical rain forests. The northern portion hosts forest with a dense, high canopy (30 m in height, with some emergent trees as high as 45 m). The lower strata are well developed with a dense understory. The southern portion hosts similar forests but with a more open canopy and less dense understory. Most trees are of small diameter (less than 300 mm) with very few stems growing to between 400 and 700 mm diameter. The most important families in the dense forests, which are typical of other Amazon Basin forests, are Leguminosae, Sapotaceae, Lecythidaceae, Moraceae, Chrysobalanaceae, Lauraceae, and Myristicaceae. The most important species in terms of density and frequency are Eschweilera alba, E. odora, Pouteria guianensis, Vantanea guianensis, Jessenia bataua, Ragala sanguinolenta, Licania apetala, and Iryanthera ulei. Four palms are common here: Astrocaryum tucuma, Jessenia bataua, Maximilliana regia, and Socratea exorrhiza. A characteristic emergent tree of this ecoregion is the purple-flowered Physocalymma scaberrim, a hardwood timber tree with red wood popular in making furniture. The palm-like Sohnreyia excelsa, which dies after it fruits once, occurs here along with the edible fruit of Couma utilis . A region of ecological tension exists in the very southern part where the moist forest interdigitates with patches of woodland savanna vegetation.

Current StatusExcept for the southern area, the Purus-Madeira moist forest ecoregion is remarkably intact. A paved road was built several decades ago along the entire length of the ecoregion from Manaus to Humaitá, but it has been closed for a decade because it was impossible to maintain in such a dynamic environment. The Transamazon Highway does bisect the southwestern end between Humaitá and Lábrea, bringing with it forest conversion to cattle ranching, agricultural fields, and colonization by small farmers. The Cuniã Ecological Station is located in the very southern reach of this ecoregion near the Madeira River, but no other protected areas exist.

Types and Severity of ThreatsControlled and uncontrolled burning threatens habitat in the southern part of this ecoregion. Cattle ranching, industrial-scale agricultural projects, and road-building all mean trouble for natural resources in the fragile soils of this ecoregion. Mining in the upper reaches of both the Purus and Madeira Rivers also destroys habitat and pollutes waterways.

Justification of Ecoregion DelineationThis interfluvial ecoregion is bound by the Purus and Solimões (Amazon) Rivers to the north and the Madeira River to the south. Linework follows the occurrence of várzea flooded forests along these rivers (IBGE 1993). The large rivers which separate this ecoregion from its neighbors act as a formidable barrier to the distributions of many species. The ecoregion is justified in that it contains a number of endemic species (Silva 1993).